The present invention relates to fiber optic rotation sensors, and particularly to a fiber optic rotation sensor employing a Sagnac interferometer with a multimode fiber sensing loop.
Fiber optic rotation sensors typically comprise a loop of fiber optic material to which light waves are coupled for propagation around the loop in opposite directions. Rotation of the loop creates a phase difference between the counter propagating waves, in accordance with the well-known "Sagnac effect", with the amount of phase difference corresponding to the velocity of loop rotation. The counterpropagating waves, when recombined, interfere constructively or destructively to produce an optical output signal which varies in intensity in accordance with the rotation rate of the loop. Rotation sensing may be accomplished by detection of this optical output signal.
Accurate detection of the rotationally induced Sagnac phase difference requires that nonrotationally induced phase differences, caused by the physical properties of the fiber (e.g. fiber birefringence), be substantially eliminated, since such phase differences are indistinguishable from the Sagnac phase difference, and thus, may provide a "phase error" in the optical output signal. If the optical paths for the counter propogating waves are identical when the loop is at rest, so that these nonrotationally induced phase differences are eliminated, the interferometer is said to be "reciprocal", while if they are not identical, the interferometer is said to be "nonreciprocal".
Nonreciprocity in fiber optic interferometric rotation sensors is caused by two factors. First, an optical fiber may support many different fundamental or spatial modes, e.g., the HE.sub.11 mode, the TE.sub.10 mode, etc., each of which has a different propagation velocity or phase velocity. (For the present discussion, a mode may be viewed as a particular optical path through the fiber.) Second, the birefringence of an optical fiber is not uniform along the fiber, and thus, coupling of light energy between the modes exists. The presence of both of these factors causes each of the counterpropagating waves to travel different optical paths around the fiber loop, so that when they are recombined, there is a phase difference therebetween. This phase difference, which may be several orders of magnitude larger than the Sagnac phase difference, is indistinguishable from the rotationally induced Sagnac phase difference, and thus, manifests itself as an error in the optical output signal. It should be emphasized that neither of the above two factors taken individually is sufficient to destroy reciprocity. Both factors must be present to yield nonreciprocal operation.
The prior art has endeavored to satisfy the reciprocity requirement by utilizing single mode fibers, which have only one fundamental propagation mode, namely, the HE.sub.11 mode. Although use of a single mode fiber would theoretically eliminate the first factor, and thus, provide reciprocity, it has been found that, in practice, a single mode fiber has two orthogonal polarization modes which are nearly degenerate, i.e., they have slightly different propagation phase velocities. Such difference in propagation velocities, while quite small, is nevertheless sufficient to cause nonreciprocal operation of single mode fiber rotation sensors. This problem has been solved in the prior art, e.g., by utilizing a fiber optic polarizer to block one of the two polarization modes of the single mode fiber as described in an article by R. Ulrich and M. Johnson, entitled "Fiber Ring Interferometer Polarization Analysis", in Optics Letters, Vol. 4, pp. 152 (April 1979).
There has been little, if any, attention directed towards the use of multimode fibers for rotation sensing, because of the large number of modes and different propagation velocities. While the difference in propagation velocities between modes can be reduced by utilizing a graded index multimode fiber, as opposed to a step index multimode fiber, the difference in propagation velocities for either type of fiber is sufficiently large that multimode fibers have heretofore been considered unsuitable for use in rotation sensors. Moreover, the problem is further complicated by the fact that, within each of the fundamental (i.e. spatial) modes of a multimode fiber, there exists a set of mode patterns, referred to herein as "generalized polarization modes", having propagation velocities which are nearly, but not exactly, equal. The difference in propagation velocities between the generalized polarization modes of a particular fundamental mode are typically on the same order of magnitude as the difference in propagation velocities between the fundamental modes of a graded index multimode fiber. Because of the many modes involved (thousands in some cases), and their associated propagation velocities, it has heretofore been considered impossible, or at least impractical, to achieve reciprocal operation in a Sagnac interferometric rotation sensor utilizing a single multimode fiber.